The invention concerns a method of localized MR spectroscopy having a sequence of radio frequency (RF) pulses for the excitation and refocussing of spins of metabolites which are to be investigated within an investigation volume, wherein the excitation is effected by means of an RF pulse of narrow bandwidth in the presence of a slice selection gradient as well as, if appropriate, an additional limitation of the investigation volume through use of refocussing pulses of narrow bandwidth in the presence of mutually orthogonal slice selection gradients.
A method of this type is known in the art from NMR in Biomedicine, Vol. 2, No. 5/6, 1989, pages 216-224.
Localized NMR spectroscopy is an important method for diagnosis, monitoring and research of various diseases. The target volume is localized in the conventional methods through the application of so-called slice-selection pulses. Thereby, in general, one pulse effects a selection in one spatial direction so that a sequence of three such pulses, in the presence of mutually orthogonal gradients, is utilized to localize a three-dimensional target volume. Depending on the type of signals produced, this corresponds to so-called STEAM (stimulated echo) or PRESS (spin echo) selection methods. An additional separation of the signal within a selected volume is usually achieved through the application of phase encoding gradients. These methods are known in the art as chemical shift imaging (CSI).
These localization methods all have the problem that execution of the required method steps leads to a time delay te between signal excitation and detection. During this time, decay of the signal to be observed occurs due to the relaxation time T2 (in case of STEAM also T1). This effect is e.g. rather small for the case of proton spectroscopy at easily achievable read-out times of te=20-30 ms, since the relaxation times of the conventionally observed metabolites lie in the range of several hundred milliseconds and therefore only a small weakening of the signal occurs.
The signal losses associated with signals of coupled substances are much more serious. In j-coupled spin systems the individually occurring multiplet signals are subject to differing coupling constant dependent dephasing. When such multiplet signals overlap, this dephasing leads to a substantial loss in the signal intensity to be observed. One can show that this signal loss e.g. for the observation of the in vivo signals of glutamate/glutamine in the proton spectrum, assumes a value of 50% for an echo time of 30 ms.
With new full body MR apparatus, shorter echo times can be realized. However, in addition to the desired metabolite signals, very wide additional signals from substances having short relaxation times in the range 1-20 ms appear in the proton spectrum. Same lead to a wide overlapping background signal which makes determination of the intensity of the desired signals extremely difficult.
It is therefore the purpose of the invention to avoid this signal loss to facilitate observation of the full intensity of the multiplet signal in proton spectra with echo times of 20-30 ms by means of which the broad signals of rapidly decaying substances have died off.